1. Field of the Invention
The present invention relates to a structure of a terminal for a battery and safety device therefor being operational when the pressure in the battery increases.
2. Description of the Conventional Art
Recently, electronic equipment such as home video decks, portable CD players and portable telephones have come into wide use. Since these electronic equipments are becoming more compact while exhibiting higher performance, efforts are also being made to reduce their size but increase the capacity of sealed batteries which supply power to the equipment. A problem with such small, sealed batteries is that once the internal pressure increases for some reason, rapid battery failure can potentially occur.
For example, if heat is generated due to the overcharging of a storage battery or if a short-circuit current flows through the load that is short-circuited due to wrong use of the battery during discharging, the electrolyte in the battery will either decompose or evaporate or, alternatively, the product of abnormal reaction due, for example, to the decomposition of electrode active materials will fill the battery space, thereby causing an abnormal increase in the internal pressure or the temperature of the battery.
To prevent heat generation or battery failure due to these abnormal behaviors, it is required that the interior of the battery is provided with a structure that senses a certain pressure in the battery in response to one of those phenomena, thereby breaking the circuit to the load.
FIG. 1 shows a conventional safety device for use with a storage battery. Shown by 341 is a case, 342 is a gasket, 343 is a valve hole, 344 is a cover, 347 is an elastic valve body, 345 is a recess in the valve body 347, 346 is a thin-walled portion of the valve body 347, 348 is a gas vent hole, 349 is a terminal plate in the form of a dish facing down, and 350 is a cutting blade.
Referring to FIG. 1, the pressure-releasing safety device is provided above the case 341 which also serves as the negative terminal. This safety device is comprised of the cover 344 which is fitted on top of the insulating gasket and which has the valve hole 343 in the center, the elastic valve body 347 in a cylindrical form that has the recess 345 and which has the thin-walled portion 346 in the bottom, and the terminal plate 349 that is fitted over the elastic valve body 347 and which has the gas vent hole 348 formed in the top. The recess 345 in the elastic valve body 347 contains the cutting blade 350 which projects toward the thin-walled portion 346. As the pressure in the battery increases, the thin-walled portion 346 of the elastic valve body 347 extends toward the cutting blade 350 until it contacts the latter. If the pressure increases further, the thin-walled portion 346 is broken by the cutting blade 350 and the gas in the battery will be released into air atmosphere via the gas vent hole 348 in the terminal plate 349, thereby ensuring against burst that would otherwise occur due to the pressure buildup in the battery.
The safety device described above prevents pressure buildup in storage batteries by breaking the safety valve, namely, the elastic valve body. However, the charging current and the discharging current will keep flowing irrespective of the pressure in the battery and, hence, the decomposition of the electrolyte and the active materials in the battery is promoted while, at the same time, the temperature of the battery will keep increasing.
With a view to solving these problems of the conventional safety device for use on the storage battery, it has been proposed in Examined Japanese Patent Publication Hei. 2-288063 that the safety valve is adapted to deform to interrupt the current flow when the pressure in the battery reaches a specified level (see FIG. 2).
Referring to FIG. 2, numeral 363 designates a gasket, 362 is an explosion-proof valve that will deform upward as the pressure in the battery increases, 364 is a lead cutting stripper fitted in contact with the underside of the explosion-proof valve 362, 365 is a closing cover, 366 is an electromotive element, 367 is an insulator, 368 is a lead plate that is fitted beneath the lead cutting stripper 364 and which is connected both to an internal terminal (not shown) on the positive electrode of the electromotive element 366 and to the explosion-proof valve 362 connected to the positive terminal, 371 is a hub around which a sheet of the electromotive element is wound, and 375 is a thin-walled portion of the explosion-proof valve 362. The lead cutting stripper 364 has a hole formed therein, through which the lead plate 368 and the explosion-proof valve 362 are connected by welding.
The operating principle of this safety device is such that if a predetermined internal pressure is reached, the explosion-proof valve deforms and the contact at the connection between said valve and the lead plate is broken to interrupt the current flow.
However, if the contact between the lead plate and the explosion-proof valve is not broken for some reason, the charging current is not cut off and the battery may eventually be damaged. Thus, one cannot necessarily say that the safety device under consideration has succeeded in achieving a marked improvement in the reliability of small, a high-capacity storage battery. Under these circumstances, it is desired to develop a safety device of higher reliability for use with a storage battery, as well as a sealed storage battery incorporating that safety device.
Furthermore, recently, accompanying with the miniaturization of electronic instruments such as a portable radio telephone device, a portable personal computer and a portable video camera, a lithium-ion secondary battery has been made practicable, which has a high energy density and is superior in light-ability.
The lithium-ion secondary battery has the maximum electromotive force of 4.1 to 4.2 V. Therefore, the material of a positive terminal is limited to stainless steel, aluminum, titanium or the like which is stable even in high voltage.
A conventional square-type lithium-ion secondary battery uses a hermetic seal method shown in FIG. 3. In FIG. 3, a metal cover 301 is mounted at an opening portion of an upper part thereof by a laser welding. The center portion of said metal cover 301 is provided with an opening into which the positive terminal 302 is inserted, and the opening is sealed by a fusion glass 303. As a result, the sealing of the inside of the battery can be maintained.
However, in the above described hermetic seal method, a large positive terminal 302 can not be assembled in the fusing glass 303. Therefore, it is difficult that an electric collecting lead is surely mounted at an end portion of the positive terminal 302 in the side of the inside of the battery. In addition, if the battery is integrated with a protection circuit and a outer terminal so as to form a battery pack, a lead for connecting to the protection circuit and outer terminal is hard to be mounted at the end portion of the positive terminal 302. Further, the fusing glass 303 is easily broken by a mechanical or heat shock.
In addition, in the hermetic method, the coefficient of thermal expansion of the positive terminal 302 has to be near that of the fusing glass 303. Accordingly, merely titanium or particular stainless steel can be used as the material of the positive terminal 302. Such a material is very expensive and has a problem with the workability thereof.
Accordingly, Unexamined Japanese Patent Publication Nos. Hei. 6-111802, Hei. 6-111803, Hei. 6-111804 and Hei. 6-111805 suggest a battery positive pin which is comprised of Al or Al-alloy and fixed to a battery can or battery cover. This battery positive pin is comprised of Al or Al-alloy so as to be superior in resistance to electric corrosion and low-cost to manufacturing.
However, since Al or Al-alloy is used for the material of the pin, it is difficult to surely connect the electric collecting lead to the pin by welding. In addition, the lead for electrically connecting to the protection circuit and outer terminal is also difficult to be mounted at the end portion of the battery cathode pin by welding.